1. Field of the Invention
This invention pertains to composites of germanate glass, particularly to barium gallogermanate (BGG) glass and aluminum oxynitride (AlON), and to a process for forming the composites by bonding the glass and the AlON.
2. Description of Related Art
Several sensors, operating in vis-IR region of 0.4-5.0 μm wavelength, currently use polycrystalline ZnS, which includes multi-spectral ZnS and CLEARTRAN®, hereinafter referred as zinc sulfide (ZnS), as a window material. ZnS is a very soft material, easily scratched and therefore not useful in rugged environments. It also has limited transmission in the visible wavelength region. Recently, new window materials have been developed, including BGG glass, Spinel and AlON, which are 3-10 times harder and 2-10 times stronger than ZnS.
Transparent AlON is a polycrystalline ceramic material, which is made by a cold isostatic pressing process followed by sintering and a hot isostatic pressing process. Large AlON windows are currently being developed for potential military applications, including sensor windows and domes on aircraft, reconnaissance windows on aircraft and unmanned airborne vehicles, missile domes and windshields for vehicles. AlON can also find many commercial applications, such as armor windows on buildings, banks, aircraft, cars, etc.
AlON is a very hard material and, consequently, its grinding and polishing costs are high. Applying a glass coating on the AlON substrate can significantly reduce its grinding and polishing costs. The index refraction of the glass and AlON can be matched to minimize reflection losses. The glass can be readily polished using traditional glass polishing techniques and therefore considerably reduce the polishing costs of AlON.
A new product is disclosed herein based on a germanate glass-AlON composite article. The new germanate glass is designed so that it can be easily bonded to an AlON substrate. The process for bonding the new glass to an AlON substrate is also disclosed. The bonded composite is very strong and withstands chemical and environmental abuse. The glass acts as an excellent dielectric layer in missile domes and window designs for applications requiring transmission in the visible, IR and microwave frequencies. For example, the composite can contain an electromagnetic interference (EMI) grid.
Many of the barium gallogermanate (BGG) glasses disclosed in the U.S. Pat. No. 5,305,414 cannot be used here because of the large coefficient of thermal expansion mismatch with AlON. Due to this large coefficient of thermal expansion mismatch, the prior art BGG glass delaminates from AlON substrates during thermal cycling. The BGG glass suitable herein bonds very well to AlON and does not delaminate.